p53 has at least three major biological functions in tumor suppression: transcriptional activation of regulatory pathways that inhibit cellular proliferation in response to abnormal growth stimuli, accumulation of p53 in response to DNA damage, and induction of apoptosis. The applicant proposes to investigate the molecular mechanisms that underlie these related functions. To simplify their studies, these investigators have divided murine and human p53 into isolated autonomous functional domains. The first 320 amino acids are sufficient for site-specific DNA binding, partial activation of p53-responsive promoters, and partial suppression of transformation by oncogenes. The C-terminal 75 amino acids are sufficient for stable p53 tetramerization and a variety of complex interactions with DNA of unknown biological significance. The proposal has five specific aims: 1. to investigate mechanisms of site specific DNA binding by p53 2. to investigate the role of p53 oligomerization and DNA looping in transactivation. 3. to investigate interactions of the C-terminal DNA binding domain of p53 with damaged DNA. 4. to identify p53 domains involved in apoptosis. 5. to select and map mutations in the p53 gene that compensate for tumor-associated defects in p53. It is expected that these genetic approaches will contribute to the understanding of how autonomous domains of p53 function and how autonomous domains interact in response to oncogenic growth stimuli and DNA damage. They may also identify new p53 functions and pathways.